The AIS is an IMO mandated system for automatically tracking ships by other ships, base stations or satellites for maritime navigation, safety and security. Vessel-mounted AIS transmitters send out AIS messages, including location information, etc., in regular time intervals. AIS receivers on ships, base stations or satellite try to correctly detect these messages. The AIS is designed for non-coherent detection. This allows for simple receiver structures at the cost of high signal-to-noise ratios (SNR) and high retransmission rates for reliable communication.
The technical characteristics of the AIS system are set out in Recommendation ITU-R M.1371-4. “Technical characteristics for an automatic identification system using time-division multiple access in the VHF maritime mobile band,” ITR-R, Tech. Rep., April 2010 (hereinafter “the AIS specification”) which is hereby incorporated by reference in its entirety.
Referring now to FIG. 1 there is shown a block diagram of a compliant AIS transmitter 100 that is used to transmit an AIS compliant message having a general field structure 10 shown in FIG. 2. The AIS specification defines a range of AIS message types that are binary sequences typically comprising one or more header fields in a message header portion followed by a data portion. The transmitter is provided 110 with an AIS message as a binary data sequence for insertion in a data payload field 20. The data payload field can have a variable length up to 168 bits. Longer messages can be split over several transmission slots. A cyclic redundancy check (CRC) block 120 calculates a 16 bit CRC value from the data portion in the AIS message 110 and the CRC value is appended to the AIS message in a 16 bit Frame Check Sequence (FCS) field 29. The FCS field will also be referred to as the CRC field (labelled FCS (CRC) in FIG. 2).
The AIS specification defines that the AIS message, and CRC are to be run length limited (RLL) to a maximum run of five consecutive “1” binary states by bit-stuffing. A bit stuffing module 130 scans the AIS message and CRC value and inserts a 0 binary state after five consecutive 1 binary states. The bit stuffed data payload field 20 and CRC field 30 are then wrapped in 8 bit start field 16 and an 8 bit end field 18, each field containing an identical 8 bit flag sequence (01111110). The flag sequence is not subject to bit stuffing even though it contains a run of six 1's. A training sequence field 14 comprising a 24 bit sequence of alternating 0's and 1's is prepended before the start flag for receiver synchronisation. A start buffer 12 is prepended before the training sequence field 14, including an 8 bit ramp-up field. An end buffer 30 is appended after the end flag field 18. The end buffer is normally 24 bits long and comprises a 4 bit bit-stuffing field 32, a 12 bit distance delay field 34, a 2 bit repeater delay field 36 and a 6 bit synchronisation jitter field 38. Note that the start buffer and end buffer are not subject to bit-stuffing.
A packet 10 is thus formed from an AIS message by adding a CRC, bit-stuffing, start and end flags, a training sequence, and start and end buffers. The packet 10 has a nominal length of 256 bits. Forming the packet can be performed by a framing block 140 which may also select the slot or slots the packet is to be transmitted in. The packet may be sent in a single slot or spread over several slots. The AIS specification defines that a transmitter (or transmitting station) may occupy a maximum of five consecutive slots for one continuous transmission, in which case only a single application of the overhead (ramp up, training sequence, flags, FCS, buffer) is required for a long transmission packet. Taking into account the 16 bit FCS field, the lengths of the data fields in the packet are 172, 428, 684, 904 and 1196 bits, respectively.
After framing, the packet is passed to a Non Return Zero Inverted (NRZI) Encoder module 150 in which a 0 at the input changes the level at the output and a 1 leaves the output unchanged. Finally this sequence is fed to a Gaussian Minimum Shift Keying (GMSK) modulator 160 with a (maximum) parameter BT=0.4. Note that the state of the NRZI encoder and of the GMSK modulator at the beginning of the packet are not specified in the AIS specification.
While the AIS system is generally effective, common sources for misdetection of an AIS signal are (a) transmission errors due to noise or other channel disturbances, and (b) interference by other AIS devices that are simultaneously transmitting.
One approach that could potentially be used to improve the AIS system would be the adoption of forward error correcting (FEC) coding principles (the AIS specification states that FEC is not used). In forward error correction (FEC) coding, redundant data is transmitted in addition to the actual data. This redundancy is exploited by the receiver to correct transmission errors. The advantages where this approach is effective are dramatically reduced error rates. The combination of FEC coding and modulation is referred to as coded modulation. FEC coding provides low error rates, while modulation provides high throughput in the case of higher-order modulation and/or bandwidth efficiency in the case of modulation with memory. Examples of coded modulation include trellis-coded modulation, multi-level-coded modulation, bit-interleaved coded modulation, and state-of-the-art schemes like LDPC-coded modulation and IRA-coded modulation.
However, simply FEC encoding the data in the AIS message given to the transmitter (ie entry point 4 in FIG. 1) and providing additional training sequences is not possible due to the following reasons. First, and most importantly, the bit-stuffing step would add bits to the codeword and thus make soft FEC decoding prohibitively complex, if not infeasible. In addition the CRC would be transmitted uncoded whilst the data would be FEC coded, and thus the CRC could not be used to detect errors. Further, the NRZI would need to be soft-decoded for high-performance iterative decoding and the NRZI/GMSK state would need to be well-defined at the beginning of any training sequence to be used for acquisition. In addition, it is desirable that the transmitter structure be designed such that existing AIS hardware and software can be reused, and such that the resulting waveform is AIS standard compliant.
There is thus a need to provide methods and apparatus improving the effectiveness of the AIS system, or at least provider users with a useful alternative.